Field of the Invention
The present invention relates to a laminate sheet structure suitable for use as electrical insulation in such things as motors and transformers.
Description of the Related Art
To those skilled in the art, the term “mica paper” refers to sheet made with high concentrations of the inorganic mineral mica, generally in an amount that is at least 90 weight percent or greater, with the remainder being a binder that provides some mechanical integrity to paper. However, the resulting mica paper is not a strong paper. U.S. Pat. Nos. 6,991,845 and 7,399,379 to Levit et al. discloses a sheet structure for electrical insulation or flame resistance comprising a barrier ply having a mica-rich face and a mica-poor face and a reinforcing ply containing a saturable backing layer attached to the mica-poor face of the barrier ply.
U.S. Pat. No. 6,991,845 to Levit et al. and U.S. Pat. No. 6,312,561 to Forsten et al. disclose an aramid-mica blended paper made from a homogenous blend of m-aramid fiber, m-aramid fibrids, and mica. Levit et al. further teaches that this “aramid-mica paper” has superior mechanical properties when compared to “mica paper” and can be used without any backing reinforcement.
In some incumbent sheet structures used in current high voltage market, high content mica paper is combined with a non-mica-containing layer of glass cloth or polyester film to compensate for mechanical weakness associated with a high mica content. However this non-mica-containing layer is believed to be responsible for most insulation failures due to its lower corona resistance, different thermal expansion, and different conductivity. Therefore electrical insulation that is capable of utilitzing more mica materials and less non-mica materials is desired.